The worst is when you walk out and it still doesn’t reset, happened to me twice now. Eventually you can wait it out. Sometimes people get in the middle of mine. Sometimes I see whole groups clearing these things. That’s part of the game unfortunately.

I agree with OP, it's very annoying when this happens.
It's already bad enough that quest "bosses" are already complete pushovers when it comes to combat - you need to seriously gimp yourself to even see any mechanics before the boss is nuked.

Unfortunately, the only usable "workaround" for this is to only do the quests at times when pretty much noone else does, such as around 1-4am GMT on PCEU.

It also does not help that some delve bosses are bugged and have extremely long respawn timers.
For example, in Nisin Cave the last boss has a ~20 minutes (!) respawn timer.

Ultimately I would really like to see all overland content to be split into a normal version (what we have now) and a veteran version (roughly on par with current normal dungeon difficulty).
This would both spread the players around more, and address the issue of overland combat being a complete joke.
Sadly it does not look like that it will be possible anytime soon, however - ESO is already on its last legs performance-wise, and I really doubt it can handle the extra load without significant investment or rework of its servers and/or codebase.